Kirimaru, whose full name is Settsuno Kirimaru, is a ten-year-old boy and one of the main students at the Ninjutsu Academy, where he is a first-year in the Ha class. He is an orphan from the province of Settsu and, as a result of losing his home and family in a war, he does not have a surname. Before enrolling in the academy, he was forced to fend for himself, earning money through any means necessary, including selling bento boxes on battlefields to pay his own tuition fees.

Kirimaru is perhaps best known for his extreme and often comical obsession with money. He is a notorious miser who is constantly thinking of ways to earn a living through a seemingly endless number of part-time jobs, which he undertakes during holidays and even during school hours. This fixation is so powerful that he can hear the sound of a coin dropping from several kilometers away, identify its value by its jingle, and even smell money. When he becomes excited by the prospect of coins, his eyes are frequently depicted turning into the shape of gold coins. He is famously weak to words like free and cheap, while words implying loss or payment displease him greatly. In one memorable instance, when asked directly whether his life or money was more important, he instantly answered coins.

Despite his greed, this personality trait is portrayed as a direct consequence of his difficult past, which forced him to be resourceful and self-reliant from a very young age. Underneath his money-driven exterior, he is fundamentally a capable, energetic, and resourceful individual. He is sharp-witted and often serves as a deadpan snarker among his friends, though his tendency to say one extra, unnecessary remark frequently lands him in trouble. Among the main trio of friends, Kirimaru is consistently portrayed as the most skilled in actual ninjutsu techniques. He is also adept at cross-dressing as a girl named Kiri-ko when a part-time job requires it, a skill at which he is unusually talented.

Kirimaru's key relationships are central to his character. He forms one-third of the main trio of friends alongside Rantarou and Shinbei, who accept his stinginess as part of his personality. More significantly, because he has no family of his own, he stays with his homeroom teacher, Doi Hansuke, during school holidays, and the two have formed a bond akin to family. Doi, who also lost his own family in the past, has taken on a paternal role, and Kirimaru is deeply sensitive to the idea of being abandoned or left alone by him. He also has positive relationships with his upperclassmen and is a member of the Library Committee.

Throughout the story, Kirimaru's motivation is primarily financial survival, but this serves as a source of both comedy and plot in the episodic series. His various part-time jobs frequently become the starting point for adventures, and his greed can sometimes be used as a weakness by enemies. However, when it comes to his friends, he often shows a more generous side, taking action and helping them without expecting payment, revealing that he values these relationships as much as, or more than, his coins. His development is subtle, showing moments of deep-seated vulnerability and a fear of loneliness that contrasts sharply with his usual energetic and money-focused demeanor, particularly in storylines that focus on his relationship with Doi-sensei. Notable abilities include his superhuman hearing for coins, high mental calculation skills for an eight-digit number, and overall proficiency in ninja techniques that rank him at the top of his class.